1 Thermal Energy, Department of Mechanical Engineering, Technical University of Denmark2 Department of Mechanical Engineering, Technical University of Denmark
This report is aimed at designing and optimizing combined cycles in order to define the most suitable machinery system for the future high-speed Incat ferry operated by Mols-Linien. For this purpose, an in-house numerical simulation tool called DNA (Dynamic Network Analysis) and a genetic algorithm-based optimization routine are used. The top cycle is modeled as the aero-derivative gas turbine LM2500, while the following five options for bottoming cycles are modeled: ∙ Single pressure steam cycle ∙ Dual-pressure steam cycle ∙ ORC using Toluene as the working fluid with an intermediate oil loop ∙ ABC with inter-cooling ∙ CO2 transcritical Rankine cycle The combined cycles are simulated with a power requirement of 18 MW, operating the gas tur-bine at part load while optimizing the bottoming cycle in regards to combined cycle efficiency. For comparison, a second scenario is simulated operating the gas turbine at full load, thus max-imizing the power output. Considering combined cycle operation with a load of 18 MW, the results suggest that the ther-mal efficiencies of the combined gas and steam cycles are 46.3 % and 48.2 % for the single pressure and dual pressure steam cycles, respectively. The combined cycles with ORC, ABC and transcritical CO2 as bottoming cycles obtained thermal efficiencies of 45.6 %, 41.9 % and 42,1%, respectively. The dual pressure steam cycle is a complex and spacious system. The single pressure steam cycle relieves some complexity and components, but has lower efficiencies as well as both sys-tems require water treatment and manual handling. The ORC is generally viewed as a less complex system, and can be automatically controlled. The pressure levels are also substantially lower than for a steam cycle, reducing mechanical stresses on components and pipes. The oil loop ensures more stable operation of the bottoming cycle during part load, compared with steam, where the reduced temperature of the heat source has adverse effect on the steam cycle power output. Still, the ORC technology is less mature, and the components are likely to be more costly as it is less developed. The ABC system applies well-known principles with low complexity, but is less competitive considering efficiency. The CO2 bottoming cycle showed re-sults comparable with the ABC solution. While the efficiencies are comparable, the CO2 cycle demands very high pressures and is not a developed technology Some preliminary evaluations of component size are done, but no conclusion could be made concerning heat exchanger dimensions. The combined cycle solution with ORC as bottoming cycle can be recommended as a machinery solution that will provide good thermal efficiencies and is less complex. While this being said, it is necessary that further work is done evaluating the component sizes for the ORC system, assuring that the weight and space requirements are held, while the thermal efficiency is kept high. If the combined cycle efficiency drops as a con-sequence of adapting the heat exchanger, the ABC system should be considered as an alterna-tive.